Gemara

(R. Ami): If Reuven put Se'or on a dough and it fermented by itself, he is liable, just like for Melachah on Shabbos.

Question: One is exempt for such Melachah on Shabbos!

(Rabah bar bar Chanah): If one left meat on coals (on Shabbos) and turned over the meat, he is liable. If he did not turn it over, he is exempt.

Answer (Rava): R. Ami is Mechayev here just like one is liable for roasting on Shabbos (i.e. when he turned it over).

(Rabah bar bar Chanah): If one left meat on coals, he is liable only if he turned it over.

Question: If the meat will not get cooked if he does not turn it over, this is obvious!

Answer #1: The meat will get cooked even if he does not turn it over.

Rejection: If so, he should be liable even if he does not turn it over!

Answer #2: If he does not turn it over, the meat will get cooked k'Ma'achal Ben Drusai (like a certain thief used to eat, a third or half cooked) on one side. (The other side will get cooked even less.) If he turns it over, it will get cooked k'Ma'achal Ben Drusai on both sides;

Rabah teaches that cooking like Ma'achal Ben Drusai on one side is insignificant.

Shabbos 18b - Version #1: One may leave (at the start of Shabbos) kid meat in a sealed (plastered) oven. One may not leave ram meat in an unsealed oven.

(Rav Ashi): One may leave kid meat (even) in an unsealed oven, or ram meat in a sealed oven;

(R. Yirmeyah mi'Difti): Both of these are forbidden.

Question (against Rav Ashi - Mishnah): One may not leave meat, onions or eggs to roast unless there is time for them to get roasted before Shabbos.

Answer: The Mishnah forbids ram meat in an unsealed oven.

Version #2: All permit kid meat, even in an unsealed oven, and ram meat in a sealed oven. They argue about ram meat in an unsealed oven. Rav Ashi permits, and Yirmeyah forbids.

Question (against Rav Ashi - Mishnah): One may not leave meat, onions or eggs to roast unless there is time for them to get roasted before Shabbos.

Answer: The Mishnah forbids meat on coals.

(Ravina): Raw gourds may be left (in an oven). Since wind is bad for it, it is like kid meat.

20a (R. Elazar): The meat, onions or eggs must be roasted k'Ma'achal Ben Drusai.

(Beraisa - Chananya): Anything cooked k'Ma'achal Ben Drusai may be left on an oven on Shabbos, even if the stove was not Garuf or Katum (the coals were not swept out, and ashes were not put on them).

Question (Mishnah): One may put Korban Pesach in an oven just before Shabbos (even if it is not cooked k'Ma'achal Ben Drusai).

Answer: The group that will eat it are zealous.

Question: This implies that if they would not be zealous, it would be forbidden. Above, we permitted leaving kid meat even in an unsealed oven! (Rav Ashi surely permits, and even R. Yirmeyah permits in Version #2.)

Answer: We permitted a kid cut into pieces (wind harms it). Korban Pesach is roasted whole (wind does not harm it).

Rishonim

Rif (Shabbos 16b): The Gemara concludes that one may not leave food on an oven that was not Garuf v'Katum unless it is fully cooked. If further cooking improves the food, e.g. cabbage, beans and ground meat, it is forbidden. One may leave any food on an oven that was Garuf v'Katum. The decree was lest one stoke coals. If they were swept out, he will not stoke them!

Ran (6b DH v'Hai): Tosfos explains that anything that reached k'Ma'achal Ben Drusai is considered cooked, and we are not concerned lest he stoke it. Chananya permits leaving it on an oven even if it is not Garuf v'Katum. Also Rav Hai Gaon says so. However, the Rif holds that anything less than fully cooked is called half cooked. He forbids even what is fully cooked, if further cooking improves it. He explains that R. Elazar permits what is k'Ma'achal Ben Drusai when it rests on the fire. Since it is already k'Ma'achal Ben Drusai, if he would stoke it, it would char. This is why the Rif brings R. Elazar's law, even though he rules unlike Chananya. The Rambam and Ramban agree.

Rambam (Hilchos Shabbos 3:16): One may roast meat, onions or eggs on a fire only if it will roast before Shabbos and be proper to eat. If it remained afterwards on the fire on Shabbos until it roasted a lot it is permitted. It is as if further cooking harms it, since if he would stoke it, it would char, since it is on the fire. This is why one may put incense under Kelim just before dark, for if he would stoke the coals, the incense would burn and smoke the Kelim.

Rosh (Shabbos 1:35): One may leave anything k'Ma'achal Ben Drusai on an oven, even if it is not Garuf v'Katum. Rabanan rule like Version #2, in which Rav Ashi permits kid and ram meat, even if the oven is not plastered. R. Yirmeyah forbids only ram meat in an unplastered oven. Rabanan rule like R. Yirmeyah. I agree, for Ravina permitted gourds because wind is bad for it, so it is like kid meat. This implies that if it were like ram meat, he would forbid unless the oven was plastered. This also implies that ram meat is forbidden if the oven was not plastered. A kid is permitted in either case, but only if it is cut up. If not, wind does not harm it, and it is forbidden. Bahag says that we discuss roasting. A support is that we brought the Mishnah about roasting.

Poskim

Shulchan Aruch (OC 254:2): One may roast an onion, egg or meat on coals only if it will roast before Shabbos on both sides k'Ma'achal Ben Drusai, which is half cooked. Even if it is kid's meat, since he left it on coals, he is concerned only that it roast quickly, even if it will char, therefore, we are concerned lest he stoke.

Beis Yosef (DH EIn): The Tur forbids leaving even raw meat near the fire close to dark. However, if it is k'Ma'achal Ben Drusai, it is permitted. The Rif and Rambam permit only if it is on the fire, for then he would not stoke lest it burn. If it is not resting on the fire, it is permitted only if further cooking harms it. Rashi and those who hold like him permit Shehiyah of anything k'Ma'achal Ben Drusai even if it is close to the fire, but not on the fire. If it is cooked k'Ma'achal Ben Drusai only on one side, in Menachos we say that this is nothing. The Rambam says so regarding Chiyuv (one who cooks it afterwards is liable), and it seems that the same applies here. There is no reason to distinguish.

Magen Avraham (7): The Mordechai says that the Gemara discusses near the coals, but not truly on coals. If it were on coals, how could he take it off (afterwards)? Perhaps they did not extinguish yet! The Rambam must explain that indeed, he does not take it until they extinguish.

Mishnah Berurah (17): It is truly on coals; he waits for them to extinguish.

Kaf ha'Chayim (19): Alternatively, he puts something on the coals, and puts the meat on a spit, and the end of the spit is on what is on the coals. Then, one may remove the meat before the coals extinguish.

Magen Avraham (9): The Mechaber forbids (if it is not k'Ma'achal Ben Drusai) even if one plastered around the cover of the oven; the Rema permits (Sa'if 5).

Kaf ha'Chayim (22): The Mechaber forbids when it is on coals even if it is plastered. Since it is on coals, it will not cool off and harden if he opens the oven.

Mishnah Berurah (17): It is forbidden only because it is on coals. If it is only near coals, the Mechaber (Sa'if 1) holds that we are not concerned let one stoke under a kid or fowl. It is soft, and stoking would char the meat. It is permitted even if it was not k'Ma'achal Ben Drusai before Shabbos. The same applies to eggs and onions. Ox or ram meat is hard; it must be fully cooked before Shabbos. If not, we are concerned lest he stoke, for this would not harm the meat if it is not resting on coals. The Rema does not distinguish on coals from near coals, or between different meats. In every case it must be k'Ma'achal Ben Drusai. If the oven is plastered shut, he permits in every case. This is our custom.

Bi'ur Halachah (DH Ein): Since eggs and onions cook quicker than kid meat, surely stoking would char them, so they are permitted whenever a kid is permitted.

Mishnah Berurah (18): Even though normally we are lenient about a kid cut into pieces (Sa'if 1), the Mechaber is stringent when it is on coals, like he explains (for he just wants it to cook quickly).

Magen Avraham (8): In YD 113:8, some say that k'Ma'achal Ben Drusai is a third cooked. Here the Shulchan Aruch rules stringently (to require half cooked for Shehiyah) due to the stringency of Shabbos.

Mishnah Berurah (16): The same applies to other things that are not eaten raw.

Kaf ha'Chayim (18): Even though one can eat a raw onion, it is better cooked.

Kaf ha'Chayim (17): Even if one roasts it to eat it tomorrow, it is forbidden, since one could stoke it and eat it at night.

Shulchan Aruch (ibid.): If it was cooked k'Ma'achal Ben Drusai, we are not concerned lest he stoke, even if it is ox meat. Since it is edible, why should he stoke to ruin it?!

Magen Avraham (10): Even if it is k'Ma'achal Ben Drusai, the Mechaber forbids if it is not resting on coals, for then one does not lose if he stokes the coals (Sa'if 1).

Gra (DH Aval): Even though the Rif and Rambam do not (normally) distinguish whether or not it is k'Ma'achal Ben Drusai, when it rests on coals, they distinguish.

Bi'ur Halachah (DH Aval): Even those (253:1) who say that k'Ma'achal Ben Drusai does not suffice for a pot, agree that it suffices for roasting.